The present invention pertains to an induced polarization logging system such as described in U.S. Pat. No. 4,359,687 by Harold J. Vinegar and Monroe H. Waxman, issued Nov. 16, 1982, and assigned to the assignee of the present invention. In this patent there is described an induced polarization logging tool and method for determining the cation exchange capacity per unit pore volume Q.sub.v, electrolyte conductivity C.sub.w, and water saturation S.sub.w of shaly sand formations using in situ measurements. In particular, the patent describes a logging tool having an insulated sonde with current and return electrodes and means to determine the in-phase conductivity, the quadrature conductivity, and the phase shift. The induced polarization logging tool which is thus described provides greatly improved means for evaluating a formation penetrated by a borehole.
The system described in the above-noted '687 patent is a "frequency-domain" induced polarization system. That is, an alternating current (AC) signal is continuously applied to the formation at a given frequency, and the resulting measurements (made either continuously or by close interval sampling) may be represented, for example, by a graph such as shown in FIG. 2. In this representation, the data corresponds to a series of points on an ellipse. As shown, the vertical offset of the center of the ellipse represents the self-potential of the formation, and the horizontal offset represents the offset current. Various frequencies can be selected from time to time, and by proper control of the AC signal input a zero current offset can even be achieved. The semi-major and semi-minor axes of the ellipse enable the in-phase and quadrature conductivity to be determined, as described in the above-noted patent.
As powerful as frequency-domain induced polarization logging has become, there are nevertheless several limitations. One, for example, is that at any given time the measurement is done with a single, discrete frequency. Another limitation is that the formation response signal is read at the same time that the stimulus voltage is applied. However, the amplitude of the out-of-phase portion of the signal is several orders of magnitude smaller than the in-phase signal portion. Typically, therefore, the amplifier gain may have to be 1,000 times less than desirable to prevent overloading in order to record both in-phase and out-of-phase portions of the signal simultaneously. This leads to an undesirably small signal-to-noise ratio for the out- of-phase or quadrature portion of the signal from which the quadrature conductivity is determined. A third limitation has to do with restrictions on the rate at which a borehole can be logged. With frequency-domain logging, particularly at lower frequencies, the tool can move an appreciable distance--perhaps well into another formation--during a single AC current cycle. Since the resistivity of the other formation may be appreciably different, and the out-of-phase voltage is measured simultaneously with the in-phase voltage, the small out-of-phase response will be substantially altered by the resistivity of the new formation.
A need thus remains for an improved method and apparatus for induced polarization logging. Ideally, such a method and apparatus will provide a substantially improved dynamic range for receiving the out-of-phase response signal, will be capable of simultaneously studying the formation response at a plurality of frequencies, and will be able to log a borehole continuously at substantially improved logging speeds.